Iran on Friday hurled new threats of retaliation against the West for the assassination of one of its nuclear scientists but also signaled a readiness to negotiate on at least one of the nuclear disputes behind the country’s worsening feud with the United States.

Even as angry throngs swarmed the memorial services for slain scientist Mostafa
Ahmadi-Roshan, state-run news media confirmed a visit to the country later this month by a special U.N. delegation to discuss alleged secret research by Iran on designing a nuclear warhead. The International Atomic Energy Agency, which will dispatch its delegation to Tehran on Jan. 28, has been pressing Iranian leaders for years to come clean about experiments.

Iran’s invitation to the IAEA was the first conciliatory gesture since the country’s leaders threatened last month to block the Strait of Hormuz in retaliation for economic sanctions. But Western diplomats and nuclear experts on Friday expressed skepticism about the meeting, noting that Iran continues to move aggressively to enlarge its stockpile of enriched uranium in defiance of U.N. and Western demands.

Olli Heinonen, the IAEA’s former top inspector, warned in an interview that Iran may be seeking to buy more time by initiating talks without freezing its production of the nuclear fuel used in weapons and at nuclear power plants.

He said Iran has taken a major step toward weapons capability with last month’s start-up of an underground plant near the city of Qom, where hundreds of centrifuge machines are making a more concentrated form of enriched uranium.

“The new machines are working,” said Heinonen, who was the U.N. agency’s nuclear safeguards chief until 2010. “By February, they will have tripled the production rate for 20 percent enriched uranium.”

Ray Takeyh, a former senior adviser to the Obama administration on the Persian Gulf region, said Iran has repeatedly sought to use negotiations as a delaying tactic.

The talks typically “will be technical and protracted,” but the “fundamental problem remains unresolved.”

“Under this cover, Iran continues to move forward,” said Takeyh, now a Middle East policy expert at the Council on Foreign Relations.

In Tehran, thousands of angry Iranians demonstrated against the United States and Israel during a burial procession for Ahmadi-Roshan, the nuclear chemist who was assassinated this week in broad daylight on a Tehran street.

“I will kill, kill those who killed my brother,” shouted the demonstrators, most of whom appeared to be members of Iran’s paramilitary Basij forces. Some held posters depicting President Obama with a Star of David on his forehead and “terrorist” written underneath.

Ayatollah Ali Khamenei, Iran’s supreme leader, also threatened revenge in a letter of condolence to the scientist’s family, which was made public Thursday. The 32-year-old chemist was described by Iranian media as the deputy director of Iran’s largest uranium-enrichment facility, near the town of Natanz.

“We will never disregard punishment for the individuals who committed this crime and the elements behind its scene,” Khamenei wrote.

Khamenei blamed the CIA and the Mossad, Israel’s spy agency, for the killing, which has added to growing tension between Tehran and Washington as Obama leads a global push for oil sanctions against the Islamic republic. On Saturday, the official Islamic Republic News Agency said Tehran has sent separate diplomatic notes to the United States and Britain insisting that both countries had an “obvious role” in the killing.

The Obama administration has denied any role in the attack and has distanced itself from the lethal tactics used. But numerous officials have warned Iran in recent days to avoid any move toward blocking the Strait of Hormuz, a vital shipping corridor for Middle Eastern oil.

State Department spokeswoman Victoria Nuland said Friday the warnings have been conveyed publicly as well as through the private, informal channels frequently used by Washington to offer its views to Iranian leaders. Iran and the United States have no formal diplomatic ties.

Also Friday, the U.S. military confirmed that it has gradually increased the number of troops based in Kuwait to 15,000, including two combat brigades, but denied that their presence was part of any military plan to engage Iran.

Navy Capt. John Kirby, a Defense Department spokesman, said the increase was temporary and related to the withdrawal of U.S. troops from Iraq last month. He declined to say how long the troops would remain in Kuwait and played down the significance of their presence.

“I want to disabuse everybody of the notion that there’s some kind of quiet increase going on, specifically aimed at some sort of contingency planning for any one country in that part of the world,” he told reporters at the Pentagon.

At the same time, Kirby acknowledged that “Iran is certainly a factor in our discussions with our allies and in our thinking about the future of the Middle East — there’s no question about it — thanks to their destabilizing behavior.”

Ahmadi-Roshan’s slaying on Monday by a bomb-wielding motorcyclist was the fourth hit in two years on an Iranian scientist with ties to Iran’s nuclear program. Security analysts have linked the attacks to a covert program that seeks to disrupt Iran’s nuclear efforts by all means short of open warfare. Iran also has witnessed a series of cyberattacks on its nuclear facilities as well as mysterious explosions at military research sites.

In the wake of the killing, some Iranian officials accused the IAEA of complicity in the attacks, saying the U.N. watchdog leaked the names of nuclear scientists to Israel so that Israel could covertly plan to kill them. The Tehran Emrouz newspaper reported Thursday that Ahmadi-Roshan had met with IAEA inspectors about his research.

“Some of the agency’s inspectors are Israeli spies, and they have given the names of our scientists to terrorist groups,” lawmaker Mohammad Karami-Rad told the semiofficial Mehr News Agency on Friday.

Current and former IAEA officials dismissed the allegation as absurd.

Warrick reported from Washington. Staff writers Craig Whitlock and Karen DeYoung and special correspondent Ramtin Rastin in Tehran contributed to this report.